Radiation beam gating and tumor tracking technologies are designed to manage tumor motion in real time during radiotherapy. The performance of these devices depends on the ability to rapidly and accurately localize the tumor. Direct tumor tracking systems are limited to electromagnetic tracking for use in the prostate exclusively because continuous x-ray imaging systems impart ionization radiation over the duration of the treatment. Thus, many systems, rather than correct due to the respiration-induced tumor motion, determine the position of the tumor from surrogates of respiration. See Hoogeman M, Prévost J B, Nuyttens J, Pöll J, Levendag P, Heijmen B. Clinical accuracy of the respiratory tumor tracking system of the Cyberknife: assessment by analysis of log files. Int. J. Radiat. Oncol. Biol. Phys. 74, 297-303 (2009); Berbeco R I, Nishioka S, Shirato H, Chen G T, Jiang S B. Residual motion of lung tumours in gated radiotherapy with external respiratory surrogates. Phys. Med. Biol. 50, 3655-3667 (2005); Vedam S S, Kini V R, Keall P J, Ramakrishnan V, Mostafavi H, Mohan R. Quantifying the predictability of diaphragm motion during respiration with a noninvasive external marker. Med. Phys. 30, 505-513 (2003); Qiu P, D'Souza W D, McAvoy T J, Ray Liu K J. Inferential modeling and predictive feedback control in real-time motion compensation using the treatment couch during radiotherapy. Phys. Med. Biol. 52, 5831-5854 (2007); Kupelian P, Willoughb T, Mahadevan A, Djemil T, Weinstein G, Jani S, Enke C, Slberg T, Flores N, Liu D, Beyer D, Levine L. Multi-institutional clinical experience with the Calypso system in localization and continuous, real-time monitoring of the prostate gland during external radiotherapy. Int. J. Radiat. Oncol. Biol. Phys. 67, 1088-1098 (2007); and Shirato H, Shimizu S, Kitamura K, Nishioka T, Kagei K, Hashimoto S, Aoyama H, Kunieda T, Shinohara N, Dosaka-Akita H, Miyasaka K. Four-dimensional treatment planning and fluoroscopic real-time tumor tracking radiotherapy for moving tumor. Int. J. Radiat. Oncol. Biol. Phys. 48, 435-442 (2000).
Methods for estimating tumor position from respiratory surrogates range from simple respiratory surrogate signal scaling to mathematically complex multi-input surrogate-based models used to determine tumor position. Regardless of its form, the surrogate-based model only remains valid while there is a constant relationship between the tumor position and the respiratory surrogate signals. However, the tumor-surrogate relationship can change during the treatment fraction, thereby causing the surrogate-based model to degrade over time. See Ozhasoglu C, Murphy M. Issues in respiratory motion compensation during external-beam radiotherapy. Int. J. Radiat. Oncol. Biol. Phys. 52, 1389-99 (2002); lonascu D, Jiang S B, Nishioka S, et al. Internal-external correlation investigations of respiratory induced motion of lung tumors. Med. Phys. 34, 3893-3903 (2007); Hugo G, Vargas C, Liang J, Kestin L, Wong J W, Yan D. Changes in the respiratory pattern during radiotherapy for cancer in the lung. Radiother. Oncol. 78, 326-331 (2006); Hoisak J D P, Sixel K E, Tirona R, et al. Correlation of lung tumor motion with external surrogate indicators of respiration. Int. J. Radiat. Oncol. Biol. Phys. 60, 1298-1306 (2004); and Malinowski K, McAvoy T J, George R, et al. Mitigating errors in external respiratory surrogate-based models of tumor position. [In press]; and Seppenwoolde Y, Berbeco R I, Nishioka S, Shirato H, Heijmen B. Accuracy of tumor motion compensation algorithm from a robotic respiratory tracking system: a simulation study. Med. Phys. 34, 2774-2784 (2007).
Currently available systems must frequently interrupt treatment to validate the surrogate-based model through additional ground-truth measurements of tumor position. The Cyberknife Synchrony™ system, for instance, validates its model at a user-selected rate of about once per minute by localizing tumor-implanted fiducials with stereoscopic radiographs. This technique of pre-scheduled intermittent data collection for model validation has at least three shortcomings: (1) if changes to the tumor-surrogate relationship occur shortly after one tumor localization, then the model can have large localization errors until the changes are detected at the next tumor localization; (2) added and unnecessary tumor localizations not leading to model updates result in unnecessary exposure to ionizing radiation; and (3) pausing for image-based tumor localization extends the duration of the treatment fraction.